Spectrophotometric devices are used to non-invasively measure biological tissue oxygenation by subjecting tissue to various wavelengths of light and observing tissue absorption characteristics. Light-emitting diodes (LEDs) are frequently used as the light source in these spectrophotometric devices because they are small, inexpensive, and available at a variety of wavelengths. LEDs can be problematic, however, because their output characteristics, such as luminous intensity and center wavelength, can change based on factors like drive current, temperature, and aging. In particular, temperature can be highly variable, especially when the spectrophotometric device is used in conjunction with techniques such as therapeutic hypothermia. As a result, the above-mentioned factors (i.e., drive current, temperature, etc.) require precise measurement or control to maintain accuracy of the spectrophotometric device. In one known control mechanism, the spectrophotometric device controls drive current to the LEDs to maintain a consistent light output from the part. This control mechanism only provides an indirect indicator of the actual light produced by the LEDs and does not monitor changes due to temperature variations.
In U.S. Pat. No. 5,477,853, Farkas and Lewis disclose methods for monitoring LED output characteristics using measurements of the forward voltage generated across the LED at various drive currents. Farkas and Lewis disclose that deviations from reference voltages were correlated with several output characteristics of the LED, including luminous intensity, temperature, and hence wavelength. As these output characteristics are based on a single measurement value (i.e., LED forward voltage), the measurements are not unique and are subject to error with increasing deviations from nominal values. It is therefore desirable to decouple the measurements and to use more direct means for monitoring the output characteristics.
What is needed, therefore, is a system for measuring the luminous intensity and temperature of a light source of a spectrophotometric device, which system decouples luminous intensity and temperature measurements and uses direct means for monitoring both output characteristics.